I'm new to Lightroom and am looking for some practical advice on exporting images from Lightroom for attachment to email and for use on the web and in social media, especially Facebook and Instagram. In particular, I'm looking for practical guidance on the "file settings" and "image sizing" fields of the export function. I've sourced a few articles on the web but all seem to offer different advice on such matters! Can someone help me with this, please?

For web/social media make sure to always use sRGB color space, jpg with a quality setting of about 60 and adjust the size in pixels as you like. Leave the file size and the resolution alone, at its default of 'not limited' and 240.

In addition to McLion's suggestions, you may want to watermark your images when publishing online. Other best practices are to look up each social media outlets specifications so that you are meeting the minimum size requirements... you can see one of many cheat sheets here: https://makeawebsitehub.com/social-media-image-sizes-cheat-sheet/

IMHO, above a quality of 90 there is not really any real gain .. except the larger filesize . I'd say for a jpg 90 is top quality. For large prints, I may use 100, but anyway prefer TIFF if the print shop accepts it.

Some sites reject your jpg if it is too large while others scale down automatically. I always resize myself to stay in control.